1. Field
The present invention relates to a fluid dynamic bearing for rotating recording disks, such as a magnetic disk and an optical disc, a spindle motor provided with the fluid dynamic bearing, and a storage apparatus provided with the spindle motor.
2. Description of the Related Art
The storage apparatus has the spindle motor that uses the fluid dynamic bearing to rotate a recording disk. The fluid dynamic bearing has a thrust bearing portion and a radial bearing portion, which are provided between a rotating member and a stationary member of the spindle motor so that a micro gap including a dynamic pressure generating groove is filled with a lubricating fluid. When the spindle motor rotates, a dynamic pressure generated in the thrust fluid dynamic bearing portion causes the rotating member to be floated with respect to the stationary member and to rotate in a non-contact state.
JP-B2-3955946 (JP-A-2004-052987) discloses a fluid dynamic bearing of a fully filled structure having only one interface. The fluid dynamic bearing has a pressure boosting portion provided at the interface side of a radial bearing portion. The pressure boosting portion induces a fluid dynamic pressure directed toward a thrust bearing portion. The fluid dynamic bearing also has a bypass passage with one end opening to a micro gap at the point where the pressure of the thrust bearing portion becomes minimum, and the other end opening to a micro gap placed at the boundary between the radial bearing portion and the pressure boosting portion.
The pressure boosting portion is configured by a spiral groove structure. The pressure boosting portion eliminates a negative pressure in an outer circumferential part of the thrust bearing portion. When the negative pressure is generated, air dissolved in a lubricating fluid is formed into air bubbles. The volume expansion of air bubbles due to the temperature rise or the like pushes up the air-liquid interface of the lubricating fluid and causes the lubricating fluid to leak out of the bearing. This affects the durability and the reliability of the spindle motor. When generated air bubbles touch the dynamic pressure generating groove, vibrations may occur. Also, the aggravation of non-repeatable run-out (NRRO) may occur. This affects the rotational precision of the spindle motor.
In a configuration described in JP-B2-3984449 (JP-A-2003-130042), a spiral dynamic pressure groove is formed on at least one of the inner circumferential surface of a sleeve and the outer circumferential surface of a shaft between the interface of a lubricating fluid and one of the radial bearing portions, which adjoins the interface of the lubricating fluid, to pump the lubricating fluid toward the thrust plate side during relative rotation and to maintain the internal pressure of the lubricating fluid, which is held in the gap located between the bearing portions, equal to or higher than atmospheric pressure.
By providing such a spiral dynamic pressure groove, a boost pressure is generated due to pumping by the spiral dynamic pressure groove, and the lubricating fluid is pumped toward the inside of the bearing device. Thus, a negative pressure is prevented from being generated in the gap located between radial bearing portions.
In a fluid dynamic bearing disclosed in JP-A-2003-156035, a plurality of regularly repeating dynamic pressure generating grooves are formed in the rotational direction of the bearing such that the grooves formed at upstream side of a lubricating fluid flow is higher in number than those formed at downstream side. The pumping action of the dynamic pressure grooves is enhanced by forming more dynamic pressure generating grooves at the upstream side than at the downstream side of the lubricating fluid flow wherein the turning point of each of dynamic pressure generating grooves is the boundary of upstream side and downstream side.
More particularly, the air-liquid interface at the upstream side of the lubricating fluid in a radial bearing portion is stabilized at the position lower than the dynamic pressure generating grooves during the rotation of rotor. When the rotor rotates, peaks and valleys of dynamic pressure generating grooves alternate in the air-liquid interface. Thus, narrow gaps and wide gaps are alternately created between the radial bearing surface and the outer circumferential surface of the rotational shaft of the rotor. Accordingly, the air-liquid interface is undulated like a pulsating-wave. Then, when the number of dynamic pressure generating grooves in the air-liquid interface is small, the number of waves is small, while the amplitudes of the waves are large. Consequently, the wave-motion of the air-liquid interface makes it easier to incorporate air bubbles in the interface.
Thus, by forming second grooves in an extension region adjacent to the dynamic pressure generating grooves, in which the air-liquid interface is located, and increasing the number of grooves in the extension region, the amplitude of the wave motion in the air-liquid interface is reduced and the capture of air bubbles at the air-liquid interface is prevented. The above facts are described in JP-A-2003-156035. That is, the second grooves are formed in a gap portion that is not filled with the lubricating fluid and provides the effect of pushing back the undulated air-liquid interface.
Spiral dynamic pressure grooves are formed consecutively at the bottom portion of lower herringbone dynamic pressure grooves. When the rotor rotates, the air-liquid interface of the lubricating fluid moves down at the position of the bottom portion and is undulated like a pulsating-wave and air bubbles are easily caught in the air-liquid interface. Thus, the spiral dynamic pressure grooves are provided a position adjacent to and lower than the interface. Consequently, the amplitude of the wave motion in the undulating air-liquid interface during rotation of the rotor is reduced by pushing back the interface toward the inner side of the bearing. Thus, air bubbles caught in the interface are reduced. Accordingly, the lubricating fluid is prevented from leaking out of the bearing.
In summary, the spiral dynamic pressure grooves described in JP-A-2003-156035 are located outside the lubricating fluid during rotation of the rotor and when the amplitude of the wave motion of the air-liquid interface is increased, so that the interface reaches the spiral dynamic pressure grooves, a pumping force of the spiral dynamic pressure grooves serves to push back the air-liquid interface.
In a configuration described in JP-A-2004-107555, lubricating fluid reservoir recesses are arranged at both ends of each of the herringbone dynamic pressure grooves formed at two places axially spaced apart on the inner circumferential surface of the sleeve of a fluid dynamic bearing. A spiral dynamic pressure groove is provided therein at the vicinity of the opening end of the bearing. In addition, annular grooves are provided between the spiral dynamic pressure groove and herringbone dynamic pressure groove and between the two herringbone dynamic pressure grooves, respectively. Consequently, the spiral dynamic pressure groove pumps the lubricating fluid to the inside of the bearing during rotation and prevents the leakage of the lubricating fluid.
JP-A-2006-275077 discloses a structure of a dynamic pressure fluid bearing device having two air-liquid interfaces arranged respectively at each side of the axial direction, in which spiral dynamic pressure grooves are formed in both ends of a bearing, respectively, to pump the lubricating fluid to the inside of the bearing during rotation. According to this reference, internal dimensions of the bearing are detailed to prevent the reduction in the dynamic pressure due to short of lubricating fluid caused by air bubbles captured when the spiral dynamic pressure grooves pump in the lubricating fluid.
A dynamic pressure is generated by the pumping action of the dynamic pressure grooves in the fluid dynamic bearing. Lubricating fluid is directed into the central part of each bearing portion and the fluid dynamic pressure is maximized at the central part of each bearing portion. On the other hand, the internal pressure of the lubricating fluid is decreased in the vicinity of both ends of the dynamic pressure groove. The decrease in the internal pressure at the vicinity of the groove end becomes more significant as the groove depth increases and the bearing gap decreases. In some cases, the internal pressure of the lubricating fluid may decrease until a level below the atmospheric pressure resulting in a negative pressure.
When the negative pressure is generated, air bubbles are easily generated in the lubricating fluid. Air bubbles may cause deterioration of the bearing performance, such as the generation of vibrations and the aggravation of NRRO (non-repeatable run-out). Thus, air bubbles may affect the rotational precision of a spindle motor. Further, when the spindle motor rotates, the temperature of the lubricating fluid rises. Thus, air bubbles that remain in the fluid dynamic bearing are thermally expanded at high temperature and may cause the lubricating fluid to leak out of the bearing device. Consequently, air bubbles may affect the durability and the reliability of the spindle motor. Therefore, air bubbles are undesirable in the lubricating fluid.
However, JP-B1-3955946 fails to describe the problem of the negative pressure generated in the vicinity of the end of the spiral dynamic pressure grooves formed in the pressure boosting portion. JP-B1-3984449 also fails to suggest about eliminating the generation of the negative pressure in the vicinity of the spiral dynamic pressure groove adjoining the lubricating fluid interface.
In the bearing described in JP-A-2003-156035, the spiral dynamic pressure groove is initially located outside the lubricating fluid, during rotation of the bearing. When the amplitude of the wave motion of the air-liquid interface is increased so that the interface reaches the spiral dynamic pressure grooves, the pumping force of the spiral dynamic pressure groove is generated to push back the air-liquid interface. However, the bearing described in JP-A-2003-156035 does not have the function of preventing the generation of the negative pressure in the lubricating fluid.
The bearing described in JP-A-2007-107555 is provided with annular grooves that are formed between the spiral dynamic pressure groove and each herringbone dynamic pressure groove and between both herringbone dynamic pressure grooves. The spiral dynamic pressure grooves pump the lubricating fluid toward the inside of the bearing and have advantages in preventing the leakage of the lubricating fluid. However, JP-A-2007-107555 does not describe the elimination of the negative pressure generated in the spiral dynamic pressure grooves.
JP-A-2006-275077 describes the structure provided with the dynamic pressure grooves that generate a dynamic pressure acting to the inside of the sleeve member during rotation of the bearing. However, no path for making the upper and lower interfaces communicate with each other and for circulating the lubricating fluid therebetween is provided in the structure. JP-A-2006-275077 discloses no means for eliminating the condition in which a negative pressure is easily generated in the vicinity of the ends of each spiral dynamic pressure groove.